It is well known that soilborne fungal phytopathogens cause enormous economic losses in the agricultural and horticultural industries. In particular, Rhizoctonia solani is one of the major fungal phytopathogens exhibiting strong pathogenicity; it is associated with seedling diseases as well as foliar diseases such as seed rot, root rot, damping-off, leaf and stem rot of many plant species and varieties, resulting in enormous economic losses. For example, Rhizoctonia solani AG 1 (IB) causes plant diseases in crops such as cucumber (Cucmumis sativus L.) and pepper (Capsicum annuum L.) as well as brown patches of golf green creeping bentgrass, Rhizoctonia solani AG 2--2 causes large patches in fairway turfgrass on large areas of golf courses resulting in enormous economic losses. In addition, Phytophthora capsici is a widespread and highly destructive soilborne fungal phytopathogen that causes root and crown rot as well as the aerial blight of leaves, fruit, and the stems of peppers (Capsicum annuum L.). It is very difficult to suppress Phytophthora blight in pepper plants if they are infected by a Phytophthora capsici. Phytophthora capsici is a particularly destructive phytopathogen responsible for blight of pepper plants during hot and humid rainy seasons resulting in the killing of the pepper plants and consequently enormous economic losses. As mentioned above, both Rhizoctonia solani and Phytophthora capsici are major fungal phytopathogens which exhibit strong pathogenicity and produce spores which survive for a long time in harsh conditions. Thus, they repeatedly cause plant diseases in large areas when conditions for incidence of the disease are optimal.
Therefore, growers generally apply a mixture of fungicides to plants on a regular basis to control plant diseases caused by both Rhizoctonia solani and Phytophthora capsici. However, it is becoming increasingly difficult to control Phytophthora blight of pepper plants and Rhizoctonia brown patches of creeping bentgrass because of the emergence of strains of fungal phytopathogens resistant to fungicides. It is also becoming increasingly difficult to control these diseases due to the widespread incidence of Rhizoctonia solani resistant to fungicides in golf courses and the proliferation of Phytophthora capsici resulting from the successive cultivation of pepper plants.
Intensive use of agrochemicals has also provoked residual toxicity and environmental problems. Furthermore, agrochemicals are considered ineffective in controlling soilborne plant diseases due to wash out, and the lack of an efficient application which allows effective penetration into rhizosphere soils. It is also very difficult to expect long term protection of plants with an application of chemical fungicides. Therefore, using certain rhizosphere bacteria as a biocontrol agent can provide not only more effective and economical practices for the control of plant diseases such as Phytophthora blight caused by Phytophthora capsici and golf course turfgrass diseases caused by Rhizoctonia solani, but also increased environmental conservation.
It has been shown that use of antagonistic microorganisms is an attractive way to control fungal pathogens (Suh, Ph.D. Dissertation. University of Idaho, Idaho USA. 1992; Crawford et al, Appl. Environ. Microbiol. 59:3899-3905, 1993; U.S. Pat. No. 5,403,584). The inventors disclosed in the prior art that certain Streptomyces species strains can be used to control plant pathogens by using a delivery medium containing peat moss, sand, and cornmeal, in the potting mixture/soil or by coating plant seeds with sodium alginate containing the biocontrol agent, resulting in the colonizing of the roots (U.S. Pat. No. 5,403,584). It has also been disclosed in the prior art that peat containing a beneficial biocontrol agent, can be used to control plant pathogens (U.S. Pat. No. 4,595,589). The delivery medium methods in the prior arts are suitable for the application of seedlings/potting mixtures, but are not quite suitable for direct application to plant seeds or plant roots to achieve efficient protection against fungal phytopathogens.
The object of the present invention is to provide novel strains of Streptomyces species which are capable of controlling fungal phytopathogens by direct application to plant seeds or plant roots utilizing specially designed formulations comprising delivery media containing a biologically pure culture selected fiom the newly isolated strains of Streptomyces species.
Further, it is an object of the present invention to provide antifungal biocontrol agents and describe their use in protecting plants from infections caused by soilborne fungal phytopathogens.